New diphosphine ligands based on heterocyclic aromatics inducing very high regioselectivity in rhodium-catalyzed hydroformylation: Effect of the bite angle

Article abstract of DOI:10.1021/om00006a057

The effect of the bite angle on regioselectivity in the rhodium-catalyzed hydroformylation reaction was studied with a series of bidentate diphosphines based on xanthene-like backbones as ligands. The bite angles of these ligands are fine-tuned by subtle alterations of the backbone of the ligands. When the bridge (X) in the 10-position of xanthene is varied, the bite angle as calculated from molecular mechanics increases stepwise from 102 to 131°, whereas the changes in steric bulk and electronic effects are virtually absent for the following ligands: bis(2-(diphenylphosphino)phenyl) ether (DPEphos, 1), X = H, H; 4,6-bis(diphenylphosphino)-10,10-dimethylphenoxasilin (Sixantphos, 2), X = Si(CH₃)₂; 2,8-dimethyl-4,6-bis(diphenylphosphino)phenoxathim (Thixantphos, 3), X = S; 9,9-dimethyl-4,6-bis(diphenylphosphino)xanthene (Xantphos, 4), X = C(CH₃)₂; 4,6-bis(diphenylphosphino)dibenzofuran (DBFphos, 5), X = bond. In the hydroformylation of 1-octene the regioselectivity increased regularly with increasing bite angle: at 40°C up to 98.3% n-aldehyde was obtained with Xantphos, without isomerization or hydrogenation of 1-octene. DBFphos does not form chelates, and consequently no increased selectivity was observed. The selectivity of the catalyst was almost unaffected by raising of the temperature to 80°C, resulting in a higher turnover frequency (tof) with a constant selectivity: 97.7% n-aldehyde, 0.5% isomerization, and a tof value of 800 mol (mol of Rh)^-1 h^-1. Xantphos induces the highest selectivity for the formation of the linear aldehyde reported for diphosphines in the hydroformylation of 1-alkenes until now. The complexes (diphosphine)Rh(H)(CO)(PPh₃) and (diphosphine)-Rh(H)(CO)₂ were prepared and identified with ¹H, ³¹P, and ¹³C NMR. The enhanced selectivity to the linear aldehyde was also observed for styrene (70% n-aldehyde with xantphos compared to 11% with triphenylphosphine). An X-ray crystal structure of the Xantphos ligand is presented (orthorhombic, space group Pbnm, with a = 8.7678(8) A?, b = 18.967(1) A?, c = 19.181(1) A?, V = 3189.8(4) A?³, and Z = 4).